CBAO National Commodity Tax, Customs and Trade Section – 2014 GST/HST Questions for Revenue Canada, Q. 34.
How does a taxpayer determine the number of non- resident attendees to attend a foreign convention if it has not yet occurred? CRA responded:
[T]here may be circumstances where it may not be reasonable to make the assumption that because the appropriate percentages were met for the previous year's event that they will be met for an upcoming event. The following is an example… :
A society, based in Portland, Oregon, USA, held a convention last year in Sydney, Australia, where only three attendees out of 100 were from Canada. The prior year's convention…in Seattle, Washington, USA, had forty-five attendees out of 100 who were from Canada. …[F]or this year's convention in Calgary, Alberta, it would not be reasonable to just use the attendance from the Australian conference… . A reasonable method…would likely consider more historical attendance data and an analysis of who attends or who does not attend their conventions based on the geographical proximity of the convention venue with the location of society members.
For a first year convention where the sponsor does not have the benefit of attendance data from prior conventions, the determination of the percentage of admissions that are reasonably expected to be supplied to non-residents should be based on the other information…[f]or example…the country of residence of the invited delegates, the country of residence of association members, [or] surveys of members' intentions… .